The present invention relates to a method of producing a hydrodynamic pressure type porous oil-impregnated bearing which is suitable for use as a bearing for small-sized motors required to have high rotational accuracy and a low torque characterstic, such as drive motors for CD-ROM, DVD-ROM, DVD-RAM, laser beam printers, hard disks, floppy disks. Zips and the like or for axial flow fan motors.
In small-sized spindle motors associated with information-handling devices, the possibility of replacing rolling bearings by sintered oil-impregnated bearings is being investigated for the purpose of cost reduction. However, since a porous oil-impregnated bearing is a kind of circular bearing, it tends to produce unstable vibrations where the shaft eccentricity is small, having the drawback of being liable to induce the so-called whirl in which the shaft is subjected to a revolving vibration at a rate which is half the rotary speed. Countermeasures therefore include the provision of hydrodynamic pressure generating grooves, such as the herringbone or spiral type, in the bearing surface. In such hydrodynamic pressure type bearings, the rotation of the shaft produces hydrodynamic pressure in the bearing clearance to support the shaft, exhibiting a high effect of suppressing shaft vibration.
In this connection, generally the porous oil-impregnated bearings are singly used. Besides this, however, there is known an arrangement wherein a shaft is supported at two places by press-fitting two porous oil-impregnated bearings into a bearing holder. With this arrangement, the bearing surface of each of the bearings can be reduced and the accuracy of each of the bearings is improved the more, the smaller the width; therefore, the arrangement is effective for uses where low torque is required or where a temperature rise should be avoided.
However, since it is difficult to secure axial alignment between two bearings, the bearing clearance has to be inevitably increased (leading to a decrease in rotation accuracy). And when hydrodynamic pressure generating grooves are formed in the bearing surface, during insert into a housing, the respective directions of the hydrodynamic pressure generating grooves of two bearings have to be made to coincide with each other, at which time there arise problems including one that it is difficult to check the direction of the hydrodynamic pressure generating grooves (since the hydrodynamic pressure generating grooves are formed in the inner peripheral surface of the bearing).
One solution to this problem is to form bearing surfaces at opposite end portions of a bearing and increase the inner diameter between the bearing surfaces to form a relief surface, methods of production thereof being known from, e.g., from Japanese Patent Publication Kokoku Showa 63-43611 and Japanese Patent Publication Kokai Heisei 2-8302.
With said two methods of production, however, it is impossible to form hydrodynamic pressure generating grooves in a bearing surfaces and hence the problems including shaft vibration cannot be solved.
Further, with the method disclosed in Japanese Patent Publication Kokoku Showa 63-43611, it is necessary to form an annular groove in an outer peripheral surface of the axially middle region of a sintered body, requiring the use of a special forming press machine or plastic processing, leading to cost increase. On the other hand, with the method disclosed in Japanese Patent Publication Kokai Heisei 2-8302, an inner peripheral surface of a powder compact is formed with a larger-outer-diameter portion and a smaller-outer-diameter portion and during the sizing operation the larger and smaller-outer-diameter portions differ in the amount of correction (the distance traveled until such portions reach a sizing core rod), so that the inner diameter and pores are liable to vary in distribution.